Change your WebApi not to use your Response class but rather an IEnumerable of Customer. Use the HttpResponseMessage response class.

Your WebAPI should only require:

[HttpGet, Route("GetAll")]
public IEnumerable<Customer> GetAllCustomers()
{
var allCustomers = repository.GetAll();
// Set a breakpoint on the line below to confirm
// you are getting data back from your repository.
return allCustomers;
}

Added code for a generic response class based on the discussion in the comments although I still recommend you don't do this and avoid calling your class Response. You should rather return HTTP status codes instead of your own. A 200 Ok, a 401 Unauthorised, etc. Also this post on how to return HTTP status codes.